The taste system develops over a prolonged period and is particularly malleable (i.e., plastic) during development. Although much is known about plasticity in other sensory systems, there is only rudimentary information about plasticity in the taste system. The overall goal of this proposal is to delineate specific morphological and functional changes to the taste system that occur after peripheral neural injury during development. The projects in this proposal build upon our recent work that describes profound alterations in peripheral and central morphology of the taste system after transection in neonatal rats of the nerves that innervate the tongue. Experiments in Aim 1 will track in vivo, degenerative and regenerative changes to the morphology of fungiform papillae after neonatal chorda tympani or lingual nerve transection. In addition, the source and pattern of the few taste buds that remain will be determined. Aim 2 will build upon our recent finding that the nerve that innervates palatal taste receptors (the greater superficial petrosal nerve) undergoes reorganization and expansion of its central terminal processes after neonatal transection of the chorda tympani nerve. Through the use of tract tracing techniques, experiments will determine the degree to which the modified greater superficial petrosal nerve invades regions of the nucleus of the solitary tract already occupied by the glossopharyngeal nerve. Aim 3 will examine the structural and functional reorganization of individual neurons of the greater superficial petrosal and the chorda tympani after neonatal chorda tympani transection. Aim 4 will determine morphological changes that occur after neonatal chords tympani transection to second-order neurons that reside in the gustatory zone of the nucleus of the solitary tract. Proposed studies will provide new information about: 1) the ability of fungiform and filiform papillae to switch phenotypes established during development, 2) the stability of the relationship between taste bud size and number of innervating neurons following severe disruption of that innervation, 3) central gustatory reorganization as it relates to interactions between different populations of afferent neurons, 4) the dynamic relationship between the structure and function of individual gustatory neurons following central reorganization, and 5) morphological changes that occur to central neurons that receive innervation from reorganized afferents. Thus, these studies will contribute not only to an understanding of taste stem plasticity, but also to an understanding of the development of nerve/target interactions.